I have played rust, and thats probably the best executed procedural generated map out there, but rust is a wilderness survival game where unturned is mostly based on scavenging. On top of that in rust all the towns and locations are the same (correct me if im wrong) and are just dotted around the map.
If unturned went down a similar route this would make the game even more repetitive than just having a few official maps. If everything was procedural generated, even towns or military bases, it would be super hard to code and set up to make it work sort of well, and would also take an immense amount of computing power to generate if its doing every single tree, house and cupboard.
It would also be hard for good progression to be worked into a procedurally generated system. Even if it did work, and the maps were always came out good and magically didnt bend your cpu over every time you made a new map, it would kill off map makers because a computer would just be doing their job. Also it would kill the ability to put lore or a story into your map which all of the official maps and most curated maps have scattered around.
Edit for your point: You could argue that randomizing the maps is a good thing, but knowing where to go and where the good loot is, along with useful hidden areas, is something that takes time to learn. If a new player and a experienced player both start off in a procedurally generated world, there wont be much of a skill gap between in terms of map sense and awarness, along with what to actually do, while in a normal map this experienced guy will proggress through the game faster than the new one. Obviously in 3.0 this is exaggerated by how you can beeline for military loot, but hopefully in 4.x that will be slowled down by loot progression.
TL:DR Non generated maps increase skill gap which is a good thing because it rewards people who play the game alot